Device selection, mainly. I have a Galaxy S8+ Unlocked, and a OnePlus 6. I'm looking to trade/sell both and buy a Note 9. Despite the fact that Samsung unlocked devices support all of the carriers in the country, they don't work on Fi. I have zero interest in the new Pixel devices.

It works well, but I'd be lying if I said it was 1:1 the same as my S8+. There are times (seemingly random) where the call will garble momentarily. It's not very often, but I'm not sure what your experience is on your S8+. I'm on a gigabit Fios connection at home, so not sure what the reason is. I got an update on my OP6 today, so that might resolve it. I would not call it bad or anything though. Definitely not a dealbreaker.

I have a OnePlus 6 and Unlocked Galaxy S8+ (both work with T-Mobile VoLTE and WiFi Calling). From what I've read, neither is allowed to roam on USC because...reasons? Or am I misunderstanding something?

Oh then this should be easy. You're basically going to use a modified version of the Samsung flashing tool, "Odin", to flash either the T-Mobile or Unlocked firmware. It's modified to ignore the model number. Both ROMs have Wifi Calling and VoLTE on T-Mobile, but the Unlocked firmware has no bloatware. Carrier firmware, ironically, gets updated faster.

Unlocked firmware will work on all carriers and reconfigure when you switch SIM cards (but no bloat or logos/branding). Samsung Pay and everything work, and it doesn't trip Knox.

All Samsung phones in the US and Canada, starting with the S7, have the same exact hardware. Only the software differs.

Just for another data point, my Pixel 1 from Verizon also works perfectly on T-Mobile. Wi-Fi calling & VoLTE work even without rebooting right after replacing SIM card. I was also able to unlock the bootloader a few months ago after a trick to do that was discovered.

When you enable Wi-Fi calling, is there anything that tells you that it's activated? For example, the Moto G5 Plus has a simple on-off toggle and when on, you have no way of knowing if it's actually activated until you place a call. Not good if roaming.

This. I have Wifi Calling on at work. Stable Wifi Connection, most calls I answer and the other side can't hear me, or the call just drops right away. Because of this I tend to stay on Network more, even when browsing the internet.